Upon his promotion from Double-A Mississippi in late May, Simmons was immediately thrust into the setup mix, earning six holds and even a save in his first eight major league appearances. Jordan Walden's return from the DL in mid-June bumped Simmons down in the pecking order, but Simmons continued to perform at a high level, with just two earned runs allowed in his first 20 outings. He began to slip shortly after the All-Star break, and a shoulder sprain and subsequent setbacks would cut short what was shaping up to be an outstanding season for the 24-year-old. Simmons was believed to be making steady progress in his recovery before the Braves announced in February that Simmons was forced to undergo Tommy John surgery and will miss the entire 2015 season.